1. Field of the Invention
The present invention relates to spraying of high viscosity silicone molding compositions and to cured silicone molds or tools formed by that process.
2. Background of the Art
Molding is a time-honored technique in which shaped articles are copied. The molding composition is applied to the surface to be copied. It is then removed from the copied surface, retaining a negative copy of that surface. Then, when a hardenable material is applied to the negative copy, a copy of the original results.
A wide variety of rigid and flexible materials have been used in the prior art for making molds. These include plasters, cements, and natural and synthetic polymer materials. These polymer materials include latex rubber, silicone, polyurethane, and the like.
For molding of complicated shapes, one preferred technique is glove molding or skin molding.
In the process of glove molding or skin molding, a flexible polymer material is applied to the surface to be molded. After this flexible polymer material cures or hardens, a "backup tool" may be built (usually in several separable pieces) around the original surface having the polymer skin applied thereto. The backup tool is removed, the flexible polymer material (which is now a glove mold) is removed from the surface, and then the glove mold is assembled inside the backup tool. The backup tool gives support to the glove mold. Molding material is then applied to the negative surface of the glove mold, to reproduce the original.
In the glove molding process, silicone is a preferred glove mold-forming material. One of the reasons silicone is particularly appropriate is its ability to reproduce even the finest detail. Other favorable properties of appropriately-formulated silicone rubber include its tensile strength (which is relatively high), its high tear strength, and its excellent elongation properties. These properties permit silicone rubber to mold complex shapes seamlessly, and then after removal of the backup tooling, to be removed from the molded article by stretching.
Unfortunately, silicone rubber is relatively difficult to apply to surfaces of any size. Conventional silicone molding compositions typically comprise a two or three part formulation. Each of these parts must be mixed prior to using. Mixing by conventional means almost inevitably results in air entrainment into the silicone. Entrained air must be removed from the uncured molding composition prior to its application to the surface to be molded. Such air removal is generally accomplished by drawing a vacuum over the silicone. After a period of several minutes to an hour or more, the silicone is degassed and ready to apply. Next, the silicone is brushed or troweled onto the surface to be copied. This is a labor-intensive and time-consuming procedure. Large objects are particularly difficult to mold. Such articles, ten or more square meters in surface area, can require days to coat with silicone.
Spraying the silicone has heretofore been an unacceptable alternative. As a general rule, sprayable materials have relatively low viscosity. Unless the material is thin enough to flow, air entrained during the spraying process remains in the sprayed silicone and causes surface defects. Silicone rubber may be sprayed, for example, as disclosed in U.S. Pat. No. 4,812,278. However, such material is typically low-viscosity which will not form suitable molds of vertical surfaces. In U.S. Pat. No. 5,196,240, sprayable silicone with incorporated thixotropic agents is disclosed in connection with the production of seamless bodysuits. However, the problems of air entrainment and spraying of high viscosity silicones are not considered. Moreover, there is no disclosure in the reference concerning mold preparation or the use of multicomponent silicones.
Accordingly, there is a need for an improved technique for applying silicone rubber to surfaces to form molds of those surfaces.